vkd3d/resource: Write null descriptors with their correct type when using mutable descriptor types.
Unfortunately this still doesn't handle the case in which a D3D12 null descriptor is accessed with a type different from that it was created with (e.g., create a CBV null descriptor and then access it as a UAV). Given e3045090 it seems that's allowed with D3D12, but with Vulkan the descriptor with mutable type still has an active type at any given time. So far I don't know how to fix the general case, but with this MR at least null descriptors stored with mutable types and accessed with their creation type should work properly.